Origin of the Trojan War
Many readers of the Iliad are surprised to find that the epic poem does not contain the myth concerning the start of the Trojan War. Nor does the poem contain some of the other famous stories from the Trojan war like the death of Achilles or the fall of Troy through the use of the Trojan horse. Since the Iliad starts "en media res" (in the middle of things) it begins during the final year of the Trojan War. Also, the Iliad is a story of Achilles' rage, not the Trojan War, so the poem begins with an account of what sparks the famous rage of Achilles and ends once Achilles releases his god-like anger. In order to fully understand the depth of the text, though, one must first understand why this war is being fought in the first place. The myth which tells of the origin of the war is called the Judgment of Paris.
During the celebration of the marriage of Peleus and Thetis, many gods were invited to share in the festivities. One lesser god was purposefully left off of the invite list: Eris, the goddess of discord. The married couple did not want to deal with the conflict that followed Eris wherever she went, especially not during their wedding celebration. During the feast which Zeus held in the married couple's honor, Eris crashed the party and tossed a golden apple onto the table in front of three goddesses: Athena, Hera, and Aphrodite. The apple was marked "To The Fairest" and was supposed to be awarded to the most beautiful of the Olympian goddesses. Each goddess thought she deserved the apple and appealed to Zeus' better judgment. Not wanting to draw the ire of the scorned goddess who weren't chosen, Zeus refused to judge the contest, so he chose a mortal for the task: the young Trojan prince Paris.
While tending to his flocks in a field near Troy, the three goddesses surprised Paris by appearing out of nowhere and informed him of his station as the judge of the beauty contest. Each of the goddesses made presentations of themselves along with bribes to try and sway Paris' decision. Hera offered Paris power over numerous city-states in the Mediterranean. Athena offered Paris wisdom along with great success in battle and everlasting fame. Aphrodite offered Paris the most beautiful woman in the world: the Greek woman Helen. Paris chose Aphrodite as the winner...he wanted Helen.
The only problem with wanting Helen was that she was already married to the Spartan king Menelaus. This did nothing to deter Paris, though, as he traveled to Sparta and stayed as a guest in Menelaus' palace. Aphrodite, as promised, aroused Helen's desire for the Trojan prince, and the two sailed off together back to Troy. Outraged by the blatant violation of Xenia (the sacred guest-host bond) and the fact that his wife was stolen, Menelaus appealed to his more powerful brother Agamemnon for help. Agamemnon gathered the most fearsome Greek army ever assembled and set sail for Troy, a city lying across the Aegean Sea on the banks of modern-day Turkey. Helen had the face that did, indeed, launch 1000 ships
This silly but accurate video made by high school students does a nice job retelling the myth of the judgment of Paris.